Exponential's BiCMOS Technology

Debunking the Bipolar VLSI Myth

Exponential Technology is exploiting recent developments in BiCMOS semiconductor process technology to create ultra-high-performance microprocessors for PCs and servers that offer twice the performance of today's microprocessors. By combining high-speed, bipolar logic with dense, low-power CMOS memory on a single chip, Exponential is able to overcome the power consumption, die-size and cost concerns usually associated with bipolar devices to develop a new class of BiCMOS microprocessors.

Historically, microprocessor designers have employed only CMOS technology in their designs. Bipolar's reputation for high power consumption and large die size (and resulting high cost) relegated this technology to smaller, simpler logic designs where the need for bipolar's extraordinary speed offset these other concerns. As a result, most microprocessor designers never learned to use bipolar logic, while most designers familiar with bipolar technology never made the transition to VLSI logic design. For most VLSI designers today, trying to build a microprocessor using bipolar technology would be like a piano player trying to play the violin. The two require fundamentally different approaches.

Bipolar's early reputation is at the root of a common misconception that it cannot compete with CMOS for creating VLSI circuits such as microprocessors. While this may have been true as recently as five years ago, dramatic improvements in bipolar process technology, the necessity of on-chip caches and ever-increasing clock rates have greatly mitigated these differences. But this old perception is so strong and has persisted for so long that today it is essentially self-perpetuating. As a result, virtually all microprocessors today are built using CMOS technology, and no one has seriously considered using bipolar technology for this application—until now.

Exponential Technology has gathered together the handful of experts that exist today who have extensive design experience with both VLSI and bipolar technologies. This unique team has enabled Exponential to be among the first to seriously consider using bipolar logic in a modern VLSI design. The recently developed ability to place a CMOS cache memory on the same chip with true bipolar logic has given Exponential's designers the critical technology they needed to complete their mission.

Years ago, the original goal of BiCMOS, a process that combines bipolar and CMOS transistors on the same chip, was to combine the best of two worlds: Bipolar technologies are much faster than CMOS and can drive more current, while CMOS is much denser and offers practically zero static power consumption. While this was a good goal in theory, creating a practical BiCMOS process proved difficult. Initial attempts at adding CMOS process steps to a bipolar process were too complex and costly, but then it was discovered that a simple form of bipolar transistor could be added economically to a CMOS process with relative ease.

Today, virtually all BiCMOS devices are fabricated starting with a CMOS process to which bipolar process steps are added. Unfortunately, this "bipolar-on-CMOS" process results in a relatively weak form of bipolar logic suitable only for building small- to medium-size logic functions and for driving long wires. The bipolar transistors created by this type of BiCMOS process are too large to be used to build complex logic such as a microprocessor core. As a result, devices built using this type of BiCMOS process, including the Intel Pentium and Pentium Pro microprocessors, make limited use of bipolar's high-speed capabilities, typically achieving only a 15 percent performance gain over equivalent CMOS-only implementations.

Exponential's designs rely on a recently developed bipolar process that allows complex bipolar logic functions and CMOS memory cells to reside on the same chip. Because this process is fundamentally a bipolar process from the start, the bipolar transistors can be made much smaller and faster, enabling them to be used for complex logic. In fact, Exponential has been able to achieve bipolar logic designs that are equal or smaller in size than equivalent CMOS circuitry while running at least three times faster than today's microprocessors.

This revelation may surprise many people who are under the impression that CMOS logic is a much more compact process than bipolar, requiring significantly less silicon real-estate per logic function than bipolar logic. But while this may be true at relatively low clock rates, in higher speed designs, CMOS transistors must be scaled up significantly to deliver enough current. In a 200-MHz microprocessor, the core CMOS logic becomes very large.

The proportion of the die used for memory also affects total die size. Today's microprocessor dies are about 50 percent memory, and this is expected to increase to as much as 75 percent over the next few years. Exponential's ability to place CMOS memory on the same chip with its core bipolar logic, combined with its ability to build bipolar logic that is the same size or smaller than CMOS logic, means it can maintain the same die size as CMOS-only or traditional BiCMOS designs.

Convinced it could control the die size and compete on price/performance, the only hurdle Exponential still needed to clear was power consumption. Exponential's designers began by determining a reasonable heat limit for their chip in a typical desktop PC configuration. They then designed their processor to stay within that "power budget." The result is an ultra-high-speed microprocessor, that, megahertz for megahertz, consumes about the same power as today's 3-volt CMOS designs.

If it were possible to run a CMOS microprocessor at the same high clock rate as an Exponential microprocessor, the two would consume essentially the same power. Alternatively, combining two or three CMOS microprocessors in a multiprocessor configuration to achieve processing power equivalent as that of an Exponential processor would also result in about the same power consumption.

Exponential Technology's microprocessor achievement will debunk the old myth that bipolar technology cannot be used to build VLSI devices. Not only does Exponential's design compete favorably with CMOS and other BiCMOS technologies when it comes to die size, cost and power consumption, but because it is built using true bipolar technology for the core logic, it can provide twice the performance of today's microprocessors.

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